Question

在以下查询中，我尝试从表中获取数据：company，classes_by_company和person。我在名为company_id的所有表中都有一个外键。在classes_by_company表中，一个类可以有一个class_status，可以是active或inactive。表person及其对应的person_status也是如此。我可以计算班级总数和总人数，在这里查找工作查询：SQFIDDLE。在下面的查询中，我尝试合并上一个查询，但现在还计算class_status=ACTIVE和person_status=ACTIVE的总数。我收到错误Not unique table/alias: 'c'。如何计算活动班级和人员的总数和总数？ SQLFIDDLE

SELECT a.id, a.company_id, a.status, c.class_status, p.person_status,
       c.total_count AS classes_per_company,
       p.total_count AS employees_per_company,
       c.active_count AS active_classes,
       p.active_count AS active_instructors
FROM company a
LEFT JOIN (SELECT company_id, COUNT(*) as total_count 
           FROM classes_by_company
           GROUP BY company_id) c
       ON a.company_id = c.company_id
LEFT JOIN (SELECT company_id, COUNT(*) as active_count
           FROM classes_by_company
           WHERE class_status = 'ACTIVE'
           GROUP BY company_id) c
       ON a.company_id = c.company_id
LEFT JOIN (SELECT company_id, COUNT(*) as total_count
           FROM person
           GROUP BY company_id) p
       ON a.company_id = p.company_id
LEFT JOIN (SELECT company_id, COUNT(*) as active_count
           FROM person
           WHERE person_status = 'ACTIVE'
           GROUP BY company_id) p
       ON a.company_id = c.company_id

表格结构：

CREATE TABLE company
    (
     id int auto_increment primary key,
     company_id int,
     status varchar(20)
    );
CREATE TABLE classes_by_company
(
 id int auto_increment primary key,
 company_id int,
 class_name varchar(20),
 class_status varchar(20) 
);
CREATE TABLE person
(
 id int auto_increment primary key,
employee_id int,
 company_id int,
 person_name varchar(20),
 person_status varchar(20)
);

Answer 1

非常小的变化......表别名（或子查询）不能相同

SELECT a.id, a.company_id, a.status, c.class_status, p.person_status,
   c.total_count AS classes_per_company,
   p.total_count AS employees_per_company,
   ca.active_count AS active_classes,
   pa.active_count AS active_instructors
FROM company a
LEFT JOIN (SELECT company_id, COUNT(*) as total_count 
       FROM classes_by_company
       GROUP BY company_id) c
   ON a.company_id = c.company_id
LEFT JOIN (SELECT company_id, COUNT(*) as active_count
       FROM classes_by_company
       WHERE class_status = 'ACTIVE'
       GROUP BY company_id) ca
   ON a.company_id = c.company_id
LEFT JOIN (SELECT company_id, COUNT(*) as total_count
       FROM person
       GROUP BY company_id) p
   ON a.company_id = p.company_id
LEFT JOIN (SELECT company_id, COUNT(*) as active_count
       FROM person
       WHERE person_status = 'ACTIVE'
       GROUP BY company_id) pa
   ON a.company_id = c.company_id

Answer 2

您的查询结构很好，您只需要为您加入的每个子查询提供不同的别名。

您也未能从class_status和person_status子查询返回c和p。

SELECT a.id, a.company_id, a.status, c.class_status, p.person_status,
       c.total_count AS classes_per_company,
       p.total_count AS employees_per_company,
       ca.active_count AS active_classes,
       pa.active_count AS active_instructors
FROM company a
LEFT JOIN (SELECT company_id, class_status, COUNT(*) as total_count 
           FROM classes_by_company
           GROUP BY company_id) c
       ON a.company_id = c.company_id
LEFT JOIN (SELECT company_id, COUNT(*) as active_count
           FROM classes_by_company
           WHERE class_status = 'ACTIVE'
           GROUP BY company_id) ca
       ON a.company_id = ca.company_id
LEFT JOIN (SELECT company_id, person_status, COUNT(*) as total_count
           FROM person
           GROUP BY company_id) p
       ON a.company_id = p.company_id
LEFT JOIN (SELECT company_id, COUNT(*) as active_count
           FROM person
           WHERE person_status = 'ACTIVE'
           GROUP BY company_id) pa
       ON a.company_id = pa.company_id

我将别名更改为ca表示活动类，而pa表示活动人员。

SQLFIDDLE

您还可以通过计算同一查询中的活动行数来减少连接数：

SELECT a.id, a.company_id, a.status, c.class_status, p.person_status,
       c.total_count AS classes_per_company,
       p.total_count AS employees_per_company,
       c.active_count AS active_classes,
       p.active_count AS active_instructors
FROM company a
LEFT JOIN (SELECT company_id, class_status, COUNT(*) as total_count, SUM(class_status = 'ACTIVE') as active_count
           FROM classes_by_company
           GROUP BY company_id) c
       ON a.company_id = c.company_id
LEFT JOIN (SELECT company_id, person_status, COUNT(*) as total_count, SUM(person_status = 'ACTIVE') as active_count
           FROM person
           GROUP BY company_id) p
       ON a.company_id = p.company_id

SQLFIDDLE

在同一个表中使用count表示两个不同的值

2 个答案: